libwacom and Huion/Gaomon devices

TLDR: Thanks to José Exposito, libwacom 2.12 will support all [1] Huion and Gaomon devices when running on a 6.10 kernel.

libwacom, now almost 13 years old, is a C library that provides a bunch of static information about graphics tablets that is not otherwise available by looking at the kernel device. Basically, it's a set of APIs in the form of libwacom_get_num_buttons and so on. This is used by various components to be more precise about initializing devices, even though libwacom itself has no effect on whether the device works. It's only a library for historical reasons [2], if I were to rewrite it today, I'd probably ship libwacom as a set of static json or XML files with a specific schema.

Here are a few examples on how this information is used: libinput uses libwacom to query information about tablet tools.The kernel event node always supports tilt but the individual tool that is currently in proximity may not. libinput can get the tool ID from the kernel, query libwacom and then initialize the tool struct correctly so the compositor and Wayland clients will get the right information. GNOME Settings uses libwacom's information to e.g. detect if a tablet is built-in or an external display (to show you the "Map to Monitor" button or not, if builtin), GNOME's mutter uses the SVGs provided by libwacom to show you an OSD where you can assign keystrokes to the buttons. All these features require that the tablet is supported by libwacom.

Huion and Gamon devices [3] were not well supported by libwacom because they re-use USB ids, i.e. different tablets from seemingly different manufacturers have the same vendor and product ID. This is understandable, the 16-bit product id only allows for 65535 different devices and if you're a company that thinks about more than just the current quarterly earnings you realise that if you release a few devices every year (let's say 5-7), you may run out of product IDs in about 10000 years. Need to think ahead! So between the 140 Huion and Gaomon devices we now have in libwacom I only counted 4 different USB ids. Nine years ago we added name matching too to work around this (i.e. the vid/pid/name combo must match) but, lo and behold, we may run out of unique strings before the heat death of the universe so device names are re-used too! [4] Since we had no other information available to userspace this meant that if you plugged in e.g. a Gaomon M106 and it was detected as S620 and given wrong button numbers, a wrong SVG, etc.

A while ago José got himself a tablet and started contributing to DIGIMEND (and upstreaming a bunch of things). At some point we realised that the kernel actually had the information we needed: the firmware version string from the tablet which conveniently gave us the tablet model too. With this kernel patch scheduled for 6.10 this is now exported as the uniq property (HID_UNIQ in the uevent) and that means it's available to userspace. After a bit of rework in libwacom we can now match on the trifecta of vid/pid/uniq or the quadrella of vid/pid/name/uniq. So hooray, for the first time we can actually detect Huion and Gaomon devices correctly.

The second thing Jose did was to extract all model names from the .deb packages Huion and Gaomon provide and auto-generate all libwacom descriptions for all supported devices. Which meant, in one pull request we added around 130 devices. Nice!

As said above, this requires the future kernel 6.10 but you can apply the patches to your current kernel if you want. If you do have one of the newly added devices, please verify the .tablet file for your device and let us know so we can remove the "this is autogenerated" warnings and fix any issues with the file. Some of the new files may now take precedence over the old hand-added ones so over time we'll likely have to merge them. But meanwhile, for a brief moment in time, things may actually work.

[1] fsvo of all but should be all current and past ones provided they were supported by Huions driver
[2] anecdote: in 2011 Jason Gerecke from Wacom and I sat down to and decided on a generic tablet handling library independent of the xf86-input-wacom driver. libwacom was supposed to be that library but it never turned into more than a static description library, libinput is now what our original libwacom idea was.
[3] and XP Pen and UCLogic but we don't yet have a fix for those at the time of writing
[4] names like "HUION PenTablet Pen"...

udev-hid-bpf: quickstart tooling to fix your HID devices with eBPF

For the last few months, Benjamin Tissoires and I have been working on and polishing a little tool called udev-hid-bpf [1]. This is the scaffolding required quickly and easily write, test and eventually fix your HID input devices (mouse, keyboard, etc.) via a BPF program instead of a full-blown custom kernel driver or a semi-full-blown kernel patch. To understand how it works, you need to know two things: HID and BPF [2].

Why BPF for HID?

HID is the Human Interface Device standard and the most common way input devices communicate with the host (HID over USB, HID over Bluetooth, etc.). It has two core components: the "report descriptor" and "reports", both of which are byte arrays. The report descriptor is a fixed burnt-in-ROM byte array that (in rather convoluted terms) tells us what we'll find in the reports. Things like "bits 16 through to 24 is the delta x coordinate" or "bit 5 is the binary button state for button 3 in degrees celcius". The reports themselves are sent at (usually) regular intervals and contain the data in the described format, as the devices perceives reality. If you're interested in more details, see Understanding HID report descriptors.

BPF or more correctly eBPF is a Linux kernel technology to write programs in a subset of C, compile it and load it into the kernel. The magic thing here is that the kernel will verify it, so once loaded, the program is "safe". And because it's safe it can be run in kernel space which means it's fast. eBPF was originally written for network packet filters but as of kernel v6.3 and thanks to Benjamin, we have BPF in the HID subsystem. HID actually lends itself really well to BPF because, well, we have a byte array and to fix our devices we need to do complicated things like "toggle that bit to zero" or "swap those two values".

If we want to fix our devices we usually need to do one of two things: fix the report descriptor to enable/disable/change some of the values the device pretends to support. For example, we can say we support 5 buttons instead of the supposed 8. Or we need to fix the report by e.g. inverting the y value for the device. This can be done in a custom kernel driver but a HID BPF program is quite a lot more convenient.

HID-BPF programs

For illustration purposes, here's the example program to flip the y coordinate. HID BPF programs are usually device specific, we need to know that the e.g. the y coordinate is 16 bits and sits in bytes 3 and 4 (little endian):

SEC("fmod_ret/hid_bpf_device_event")
int BPF_PROG(hid_y_event, struct hid_bpf_ctx *hctx)
{
	s16 y;
	__u8 *data = hid_bpf_get_data(hctx, 0 /* offset */, 9 /* size */);

	if (!data)
		return 0; /* EPERM check */

	y = data[3] | (data[4] << 8);
	y = -y;

	data[3] = y & 0xFF;
	data[4] = (y >> 8) & 0xFF;

	return 0;
}
  

That's it. HID-BPF is invoked before the kernel handles the HID report/report descriptor so to the kernel the modified report looks as if it came from the device.

As said above, this is device specific because where the coordinates is in the report depends on the device (the report descriptor will tell us). In this example we want to ensure the BPF program is only loaded for our device (vid/pid of 04d9/a09f), and for extra safety we also double-check that the report descriptor matches.

// The bpf.o will only be loaded for devices in this list
HID_BPF_CONFIG(
	HID_DEVICE(BUS_USB, HID_GROUP_GENERIC, 0x04D9, 0xA09F)
);

SEC("syscall")
int probe(struct hid_bpf_probe_args *ctx)
{
	/*
	* The device exports 3 interfaces.
	* The mouse interface has a report descriptor of length 71.
	* So if report descriptor size is not 71, mark as -EINVAL
	*/
	ctx->retval = ctx->rdesc_size != 71;
	if (ctx->retval)
		ctx->retval = -EINVAL;

	return 0;
}

Obviously the check in probe() can be as complicated as you want.

This is pretty much it, the full working program only has a few extra includes and boilerplate. So it mostly comes down to compiling and running it, and this is where udev-hid-bpf comes in.

udev-hid-bpf as loader

udev-hid-bpf is a tool to make the development and testing of HID BPF programs simple, and collect HID BPF programs. You basically run meson compile and meson install and voila, whatever BPF program applies to your devices will be auto-loaded next time you plug those in. If you just want to test a single bpf.o file you can udev-hid-bpf install /path/to/foo.bpf.o and it will install the required udev rule for it to get loaded whenever the device is plugged in. If you don't know how to compile, you can grab a tarball from our CI and test the pre-compiled bpf.o. Hooray, even simpler.

udev-hid-bpf is written in Rust but you don't need to know Rust, it's just the scaffolding. The BPF programs are all in C. Rust just gives us a relatively easy way to provide a static binary that will work on most tester's machines.

The documentation for udev-hid-bpf is here. So if you have a device that needs a hardware quirk or just has an annoying behaviour that you always wanted to fix, well, now's the time. Fixing your device has never been easier! [3].

[1] Yes, the name is meh but you're welcome to come up with a better one and go back in time to suggest it a few months ago.
[2] Because I'm lazy the terms eBPF and BPF will be used interchangeably in this article. Because the difference doesn't really matter in this context, it's all eBPF anyway but nobody has the time to type that extra "e".
[3] Citation needed

Enforcing a touchscreen mapping in GNOME

Touchscreens are quite prevalent by now but one of the not-so-hidden secrets is that they're actually two devices: the monitor and the actual touch input device. Surprisingly, users want the touch input device to work on the underlying monitor which means your desktop environment needs to somehow figure out which of the monitors belongs to which touch input device. Often these two devices come from two different vendors, so mutter needs to use ... */me holds torch under face* .... HEURISTICS! :scary face:

Those heuristics are actually quite simple: same vendor/product ID? same dimensions? is one of the monitors a built-in one? [1] But unfortunately in some cases those heuristics don't produce the correct result. In particular external touchscreens seem to be getting more common again and plugging those into a (non-touch) laptop means you usually get that external screen mapped to the internal display.

Luckily mutter does have a configuration to it though it is not exposed in the GNOME Settings (yet). But you, my $age $jedirank, can access this via a commandline interface to at least work around the immediate issue. But first: we need to know the monitor details and you need to know about gsettings relocatable schemas.

Finding the right monitor information is relatively trivial: look at $HOME/.config/monitors.xml and get your monitor's vendor, product and serial from there. e.g. in my case this is:

  <monitors version="2">
   <configuration>
    <logicalmonitor>
      <x>0</x>
      <y>0</y>
      <scale>1</scale>
      <monitor>
        <monitorspec>
          <connector>DP-2</connector>
          <vendor>DEL</vendor>              <--- this one
          <product>DELL S2722QC</product>   <--- this one
          <serial>59PKLD3</serial>          <--- and this one
        </monitorspec>
        <mode>
          <width>3840</width>
          <height>2160</height>
          <rate>59.997</rate>
        </mode>
      </monitor>
    </logicalmonitor>
    <logicalmonitor>
      <x>928</x>
      <y>2160</y>
      <scale>1</scale>
      <primary>yes</primary>
      <monitor>
        <monitorspec>
          <connector>eDP-1</connector>
          <vendor>IVO</vendor>
          <product>0x057d</product>
          <serial>0x00000000</serial>
        </monitorspec>
        <mode>
          <width>1920</width>
          <height>1080</height>
          <rate>60.010</rate>
        </mode>
      </monitor>
    </logicalmonitor>
  </configuration>
</monitors>
  

Well, so we know the monitor details we want. Note there are two monitors listed here, in this case I want to map the touchscreen to the external Dell monitor. Let's move on to gsettings.

gsettings is of course the configuration storage wrapper GNOME uses (and the CLI tool with the same name). GSettings follow a specific schema, i.e. a description of a schema name and possible keys and values for each key. You can list all those, set them, look up the available values, etc.:

    $ gsettings list-recursively
    ... lots of output ...
    $ gsettings set org.gnome.desktop.peripherals.touchpad click-method 'areas'
    $ gsettings range org.gnome.desktop.peripherals.touchpad click-method
    enum
    'default'
    'none'
    'areas'
    'fingers'
  

Now, schemas work fine as-is as long as there is only one instance. Where the same schema is used for different devices (like touchscreens) we use a so-called "relocatable schema" and that requires also specifying a path - and this is where it gets tricky. I'm not aware of any functionality to get the specific path for a relocatable schema so often it's down to reading the source. In the case of touchscreens, the path includes the USB vendor and product ID (in lowercase), e.g. in my case the path is:

  /org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/

In your case you can get the touchscreen details from lsusb, libinput record, /proc/bus/input/devices, etc. Once you have it, gsettings takes a schema:path argument like this:

  $ gsettings list-recursively org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/
  org.gnome.desktop.peripherals.touchscreen output ['', '', '']

Looks like the touchscreen is bound to no monitor. Let's bind it with the data from above:

 
   $ gsettings set org.gnome.desktop.peripherals.touchscreen:/org/gnome/desktop/peripherals/touchscreens/04f3:2d4a/ output "['DEL', 'DELL S2722QC', '59PKLD3']"

Note the quotes so your shell doesn't misinterpret things.

And that's it. Now I have my internal touchscreen mapped to my external monitor which makes no sense at all but shows that you can map a touchscreen to any screen if you want to.

[1] Probably the one that most commonly takes effect since it's the vast vast majority of devices

New gitlab.freedesktop.org 🚯 emoji-based spamfighting abilities

This is a follow-up from our Spam-label approach, but this time with MOAR EMOJIS because that's what the world is turning into.

Since March 2023 projects could apply the "Spam" label on any new issue and have a magic bot come in and purge the user account plus all issues they've filed, see the earlier post for details. This works quite well and gives every project member the ability to quickly purge spam. Alas, pesky spammers are using other approaches to trick google into indexing their pork [1] (because at this point I think all this crap is just SEO spam anyway). Such as commenting on issues and merge requests. We can't apply labels to comments, so we found a way to work around that: emojis!

In GitLab you can add "reactions" to issue/merge request/snippet comments and in recent GitLab versions you can register for a webhook to be notified when that happens. So what we've added to the gitlab.freedesktop.org instance is support for the :do_not_litter: (🚯) emoji [2] - if you set that on an comment the author of said comment will be blocked and the comment content will be removed. After some safety checks of course, so you can't just go around blocking everyone by shotgunning emojis into gitlab. Unlike the "Spam" label this does not currently work recursively so it's best to report the user so admins can purge them properly - ideally before setting the emoji so the abuse report contains the actual spam comment instead of the redacted one. Also note that there is a 30 second grace period to quickly undo the emoji if you happen to set it accidentally.

Note that for purging issues, the "Spam" label is still required, the emojis only work for comments.

Happy cleanup!

[1] or pork-ish
[2] Benjamin wanted to use :poop: but there's a chance that may get used for expressing disagreement with the comment in question